1. Field of the Invention
This invention relates to a scanning tunneling microscope (STM) which are capable of measuring the shape of the surface of an electrically conductive sample with an atomic-level resolution using the tunneling effect of electrons and a spectroscopic information detection method.
2. Description of the Related Art
In the STM, an STM image of a sample is detected by scanning a pointed probe while a tunnel current locally flowing between the sample and the tip end of the pointed probe is kept constant.
The STM image contains two types of information including microscopic roughness of the surface of the sample and microscopic variation in the physical property of electrons therein. Therefore, if the latter information is constant for the samples, the STM image will represent the unevenness of the surface of the sample. A method for creating an image indicating the distribution of the electronic physical properties on the surface of the sample while the probe is controlled by the STM is called a scanning tunneling spectroscopy (STS).
When a pointed probe is set at a distance of approx. 10 .ANG. from the surface of an electrically conductive sample with a bias voltage V.sub.T applied to the surface of the sample, a tunnel current I.sub.T starts to flow. In the STM, an STM image indicating the surface condition of the sample can be obtained by scanning the probe along an XY plane parallel to the surface of the sample while the probe is being finely moved by means of a servocontrol in a Z direction so as to keep the tunnel current unchanged and recording the movement of the probe in the Z direction. The tunnel current I.sub.T exhibits the following exponential function with respect to a distance S between the probe and the sample: ##EQU1## where B denotes a numerical coefficient (approx. 1.025.ANG..sqroot.eV), R.sub.T denotes a tunnel resistance, and .phi. denotes a tunnel barrier height (.phi.=(.phi.1+.phi.2)/2, .phi.1 is a work function of the probe and .phi.2 is a work function of the sample). The tunnel barrier height .phi. for the clean metal surface is approx. 1 to 5 eV, and the tunnel current I.sub.T may vary by one digit according to the equation (1) when the distance S varies by 1 .ANG.. In the STM, a servo operation for keeping the tunnel current constant is effected by using variation in the tunnel current I.sub.T to detect variation in the distance S between the probe and the sample and finely moving the probe or sample in the Z direction by means of the finely moving element of a piezoelectric member to control the distance S. The control operation can attain the precision of less than 1 .ANG., and a 3-dimensional microscopic image or STM image of an atomic-size order can be obtained by recording a control voltage and the movement of the probe in the XY directions.
In a case wherein a sample in which the tunnel barrier height .phi. and tunnel resistance R.sub.T appearing in the equation (1) are constant irrespective of the position is used, the derived STM image will represent the unevenness of the surface of the sample.
However, in many samples among those to be actually measured, the tunnel barrier height .phi. and tunnel resistance R.sub.T of tend to locally vary. The STM image for such samples contains information representing the unevenness of the surface of the sample and information representing difference in the energy state of tunneling electrons in different positions.
As a method for separating these items of information of the STM image, there are provided a method (differential conductance method) for deriving the distribution of density of states of electrons and phonons based on the dependency of the tunnel current I.sub.T on the bias voltage V.sub.T such as .differential.I.sub.T /.differential.V.sub.T, .differential.I.sub.T /.differential.VT/I.sub.T /V.sub.T or .differential..sup.2 I.sub.T /.differential.V.sub.T.sup.2 and a method (barrier height method) for microscope for effecting the above methods is generally called a scanning tunneling spectroscope (STS).
The reason why the density of states of electrons and the tunnel barrier height can be derived by means of the STS respectively based on .differential.I.sub.T /.differential.V.sub.T and .differential.lnI.sub.T /.differential.S as described above is as follows The equation (1) of the tunnel current I.sub.T used for explaining the principle of the above STM is derived on the assumption that the tunnel barrier height .phi. can be kept constant.
However, on the actual surface, the tunnel barrier height .phi. and the probability of existence of tunneling electrons are not uniform. When this is taken into consideration, the tunnel current I.sub.T can be derived by the following equation. ##EQU2## [Rev.Sci. Instrum. 60(2), February 1989, p165] where .rho.(E) indicates the local state density on the surface of the sample, V.sub.T indicates the bias voltage of the sample with respect to the probe, and T(E,eV.sub.T) indicates the tunneling probability of electrons with energy E and can be expressed as follows: ##EQU3##
It is understood from the above description that .rho.(E) can be derived by differentiating I.sub.T with respect to V.sub.T (.differential.I.sub.T /.differential.VT.varies..rho.(E)).
Further, .phi. can be derived by differentiating the logarithm of I.sub.T with respect to S according to the equation (1) or (2) (.differential.lnI.sub.T /.differential.S.varies..sup.1/2).
This invention relates to an STS. First, the differential conductance method which is effected in the prior art is explained and then the barrier height method is explained.
A conventional case (1): A tunnel differential conductance measuring method by lock-in detection (IBM, J. Res. Develop Vol 30 No. 4, July 1986, pp. 411 to 416).
In the device shown in FIG. 1, a servo operation is effected to keep constant a tunnel current flowing when a probe 2 is set near a sample 1 while a D.C. bias voltage Vo from a D.C. bias voltage generator 6 is being applied between the sample 1 and the probe 2. At this time, in order to obtain local spectroscopic information on the surface at the tip end of the probe 2, the differential conductance of the tunnel current is measured For this purpose, an A.C. modulated (.DELTA.V.sub.T cos.omega.t) signal which is minute in comparison with the bias voltage is supplied from a sine wave generator 7 and superposed on the bias voltage. The tunnel current I.sub.T obtained at this time can be expressed as follows with attention given to modulated signal components. ##EQU4## where .DELTA.V.sub.T is a known value, and a partial differential conductance, in the D.C. bias voltage Vo, that is, (.differential.I.sub.T /.differential.V.sub.T).vertline.VT=V.sub.o can be measured by detecting the amplitude of the .omega. component appearing in the tunnel current by use of a lock-in amplifier 8.
Further, in FIG. 1, a reference numeral 3 denotes a pre-amplifier for detecting the tunnel current, and a symbol 4 denotes an XY direction finely driving mechanism for controlling a distance between the probe 2 and the sample 1 and scanning the probe 2.
A conventional case (2): LTTM (Low Temperature Tunneling Microscopy) (Phys. Rev. Lett. Vol. 54 No. 22. June 3, 1985, pp. 2433 to 2436).
In the above conventional case, attention was paid to the fact that, in a case where an attempt is made to measure the distribution of superconductive state on the surface of a superconductive sample by use of an STS operated at low temperatures, the value of the differential conductance (dI/dV) in the tunnel junction portion in the bias voltage of 0 V becomes 0 when an SIN (superconductor/insulator/normal conductor) junction is used as shown in FIG. 2A and the value is set to a limited value when an NIN (normal conductor/insulator/normal conductor) junction is used as shown in FIG. 2B, and then an attempt was made to measure the distribution of superconductive state by deriving the differential conductance when the bias voltage is set at 0 V.
In a device shown in FIG. 3, a servo operation is effected in which the probe 2 is set near the sample 1 so as to cause a tunnel current to flow while a triangular wave voltage from a bias voltage generator 6 is being applied between the sample I and the probe 2, the amplitude of the tunnel current is detected by the lock-in amplifier 8 and the amplitude of the tunnel current is kept constant by a servo circuit 5. The differential conductance value is derived by differentiating the tunnel current with respect to time by a differentiating circuit 10 (since the bias voltage is a triangular wave, .vertline..differential.VT/.differential.t.vertline. is constant. Therefore, the differential conductance=.vertline..differential.I.sub.T /.differential.V.sub.T .vertline.=.vertline..differential.I.sub.T /.differential.t .vertline./.vertline..differential.V.sub.T /.differential.t .vertline..varies..vertline..differential.I.sub.T /.differential.t.vertline.). While the servo control operation is being effected, an image of unevenness is derived by scanning the probe in the XY directions At the same time, as shown in FIG. 4, the distribution image of the differential conductance for the bias voltage of 0 V is derived by detecting a differential conductance obtained when the bias voltage is set to 0 V by using a zero-cross detector 11 and a shot pulse generator 12 and sampling the same by using a sample-hold amplifier 13 in connection with the XY scanning operation.
A conventional case (3): CITS (Current Imaging Tunneling Spectroscopy) method (Phys. Rev. Lett. 56, 18, May 5, 1986, pp 1972 to 1975, J. Vac. Sci. Technol. A6(2), Mar/Apr. 1988, pp344 to 348).
This method is to measure the distribution of local state density of surface electrons based on the dependency of the tunnel current I.sub.T on the bias voltage V.sub.T. This method is based on the fact that a differential conductance .differential.I.sub.T /.differential.V.sub.T is proportional to the local state density if a tunnel gap S and a barrier height .phi. are constant irrespective of the position on the surface of the sample.
In the CITS method, the local current and voltage values are individually stored for each position while the probe is being scanned and then the differential conductance is derived by numerical calculations. The measurement of the current-voltage characteristic in the CITS method is effected using a construction shown in FIG. 5 at timings shown in FIGS. 6A to 6E.
When a bias voltage shown in FIG. 6D which is output from a D/A converter 20 is a D.C. voltage, a fixing signal is interrupted as shown in FIG. 6A so as to set an STM servo system 22 into a servo condition by means of an ON-OFF signal generator 21 of the STM servo system and a Z direction finely moving control is effected by means of the finely driving mechanism 4 to keep the tunnel current constant. The STM servo voltage is converted into a corresponding digital signal at timings shown in FIG. 6B by means of an A/D converter 23 and recorded on a recording device or display device (not shown). Next, as shown in FIG. 6A, the servo system 22 is interrupted and the probe-sample distance is kept unchanged. In this condition, a bias voltage output from a D/A converter 20 as shown in FIG. 6D is scanned, and an A/D converter 24 is operated at timings as shown in FIG. 6C to convert the tunnel current output varying as shown in FIG. 6E into a corresponding digital signal and record the same on a recording device or display device (not shown). After this, the bias voltage is set again to the initially set value to effect the Z axis control. A sequence of operations based on the time sequence of FIG. 6 are repeatedly effected at respective points of the XY scanning voltages to simultaneously record normal unevenness data and local current-voltage characteristic data.
A symbol 25 in FIG. 5 denotes a data processor for subjecting the current data to numerical calculation processes to construct the local state density.
A conventional case (4): Barrier Height Spectroscopy (IBM, J. RES. DEVELOP. Vol. 30, No. 4, July 1986, pp355 to 369, Phys. Rev. Lett. Vol. 60, No. 12, March 21, 1988 pp1166 to 1169).
In a device shown in FIG. 7, a bias voltage V.sub.T is applied between the probe 2 and the sample 1, a distance (tunnel gap S) between the tip end of the probe and the sample is set less than several nm, a current flowing at this time is detected by an I/V converter 3, and the distance S is servo-controlled by using a Z axis finely driving mechanism of the X, Y and Z finely driving mechanism 4 so as to permit the tunnel current to be kept constant by means of the servo circuit 5. When the probe is scanned in the XY directions by the X, Y and Z finely driving mechanism 4 while the servo operation is being effected, information representing the unevenness of the surface of the sample can be derived based on a servo output signal In a case where the servo operation cannot follow variation in the servo output signal of the STM (for example, at a speed of 1/5 of the time constant), a minute modulated signal (.DELTA.Scos.omega.t) having the known amplitude is added thereto and applied to the Z axis finely driving mechanism. An application of the modulated signal (.DELTA.Scos.omega.t) causes a modulated component of the same frequency to be introduced in a tunnel current flowing between the sample and the probe.
By deriving the logarithm of the tunnel current I.sub.T using the equation (1), the following equation can be obtained. ##EQU5##
In this case, the term "const" component means that it takes substantially the same value in respective periods of time of the modulated component .omega.. Therefore, the value of barrier height .phi. can be determined by deriving .omega. modulated amplitude (B.phi..sup.1/2 .DELTA.S) by a modulated component detector 30. That is, the barrier height .phi. can be derived at the same time as the unevenness information derived from the servo output signal is obtained. In the drawing, a symbol 31 denotes an applied signal oscillator and a symbol 32 denotes a logarithm amplifier.
The defect of the conventional case (1)
In the measurement by a method in which a minute A.C. modulated signal is superposed on the D.C. bias voltage, a differential conductance at a point at which the D.C. bias value Vo is set is derived It is necessary to stably effect the probe servo operation with the set bias voltage, however, a bias voltage with which the servo operation cannot be stably effected for some surface conditions of the sample or probe is present, making it impossible to continuously measure the dependency on the bias voltage. Further, the position at which the bias voltage can be measured at one time is limited to one point and therefore the dependency of the local spectroscopic data on the bias voltage cannot be measured on the real time basis.
Further, in the servo operation effected by a D.C. bias voltage, the height of the probe and the amount of associated tunnel current may vary according to a difference in the bias voltage setting value and tunnel current setting value, and even if the dependency of the differential conductance on the bias voltage is measured at the same point, the tunnel condition such as the sample-probe distance may be changed when the bias voltage setting value is changed, thus making it impossible to measure the dependency thereof on the bias voltage under the same condition.
The defect of the conventional case (2)
In the measurement of the distribution of the superconductive state, an attempt is made to measure the superconductive state with much stress put only on the value of the differential conductance obtained when the bias voltage is 0 V. In order to determine the superconductive state, it is necessary to obtain information of the differential conductance at points other than 0 V by, for example, measuring the differential conductance at a bias voltage corresponding to a superconductive gap voltage at which the differential conductance rapidly varies. Further, when a general material is used, it becomes necessary to set a characteristic bias voltage inherent to the material and derive the differential conductance at the thus set bias voltage. However, with this method, it is only possible to measure the differential conductance set when the bias voltage is at 0 V.
Further, in order to prevent the servo operation from being influenced by harmonic components included in the bias voltage and becoming unstable, it is necessary to add a sine wave to the bias voltage, or in order to measure the dependency thereof on the response speed, it is necessary to apply a sawtooth wave. In this way, it is required to freely set the bias voltage waveform according to the measurements. However, in this measuring method, the application waveform must be a triangular waveform to measure the differential conductance.
The defect of the conventional case (3)
Since time sharing operation is effected and the servo operation is repeatedly set ON and OFF for each measuring point to record values of the current and voltage at each measuring point, the probe control system will response in a stepwise manner when the servo operation is set ON or OFF, thereby causing resonance or instability in the system. Particularly, when the servo operation for a distance between the sample and the probe is unstable, it becomes impossible to effect the STM and STS measurements.
Further, since spectroscopic data is derived by effecting the numerical operation such as differentiation based on the stored current and voltage, the number of data necessary for deriving one item of information becomes large, thus making it necessary to store a larger number of current values at each image point. For example, in a typical CITS operation, STM data is recorded on 128.times.128 points and more than 16 current-voltage characteristic data are recorded on each point. In this case, an STM data file of 16K points and an STS file having data of 256K points are necessary and therefore the number of data is increased.
Further, since differential spectroscopic information must be subjected to a numerical operation process after it has been received, the differential spectroscopic information cannot be displayed during the measurement on the real time basis.
The defect of the conventional case (4)
Since a method of comparing the modulated amplitudes to derive a differential value, the differential value cannot be derived if the application amplitude is not minute. Therefore, the application amplitude is limited, thus restricting the measurement range in the Z direction.
Further, since the modulated amplitude is minute, the relation between the barrier height .phi. and tunnel gap S cannot be measured over a wide range, and particularly, when the relation between the lnI.sub.T and S is nonlinear, the barrier height .phi. cannot be precisely derived.